This invention relates to a magnetization pattern of a stepping motor. More particularly, the invention relates to a magnetization pattern of a stepping motor for reducing vibration and noise that occur during driving of the motor by changing a magnetization balance of magnetic poles in a rotating direction of a rotor magnet.
The stepping motor of this kind has been used appropriately in the past as an intermittent driving source of various kinds of appliances such as a printer, a facsimile, a disk drive, and so forth. In the stepping motor, a rotor is arranged rotatably inside a stator having a pole tooth, and the rotor has at its outer peripheral portion a cylindrical rotor magnet formed of a permanent magnet. When the stepping motor serially switches the supply of power to excitation coils of the stator magnetic poles, the rotor Is rotated and driven relative to the stator.
In the rotor magnet 1, N poles and S poles are alternately magnetized in multiple poles at a plurality of positions and with an equal spacing among them in a circumferential direction of the rotor magnet 1 as shown in FIG. 3. The plurality of magnetization portions 2 is formed into a mutually equal arcuate magnetization width C. Incidentally, in the drawing, when the total number of poles of the rotor magnet 1 is y and a center angle of the arcuate equal width C of the magnetization portions 2 is x°, they satisfy the relation x°=360°/y.
When the rotor magnet 1 comes off from a stability point during non-supply of power in the stepping motor, a retaining torque, that is, a detent torque, for returning the rotor magnet 1 to the stability point occurs between the rotor magnetic poles and the pole teeth as shown in FIG. 4. As performances of various appliances described above have become higher in recent years, a stepping motor having a lower detent torque has been required. When the detent torque of the motor is great, a torque ripple and a rotating speed ripple of the motor increase with the result of the occurrence of problems such as vibration and noise. When the stepping motor is used for a positioning motor, positioning accuracy drops.
To reduce the detent torque, a magnetization pattern of a motor is known in which a center of each magnetic pole of a magnetization pattern at a half portion of the rotor magnet 1 in a rotating direction of a stepping motor is deviated by ¼ of a magnetization pitch from the center of each magnetic pole of the magnetization pattern of the other half portion of the rotor magnet 1, and a discontinuous portion occurring in the proximity or the boundary between these two half portions is formed into a non-magnetization portion (See, for example, JF-A-Z-114848).
The magnetization pattern of the related stepping motor described above can offset the detent torque as the phase of the detent torque occurring in the other half portion is inversed with respect to the phase of the detent torque occurring in one of the half portions. However, because the discontinuous portion occurring in the proximity of the boundary of the two half portions is formed into the non-magnetization portion, this non-magnetization drastically lowers the driving torque during driving and the stepping motor is not suitable for the practical application.